Enantiopure Phosphine Oxides with Electron‐Rich Chiral Rhenium Stereocenters: Syntheses, Structures, and Brønsted Basicities

The chiral racemic methyl complex (η 5 ‐C 5 R 5 )Re(NO)(PPh 3 )(CH 3 ) ( 2 ) is converted to the rhenium‐containing phosphorus donor ligands (η 5 ‐C 5 R 5 )Re(NO)(PPh 3 )((CH 2 ) n PR' 2 ) ( n /R/R’ = a , 0/H/Ph; 4b , 0/H/ t ‐Bu; 7a , 0/Me/Ph; 10a , 1/H/Ph; 10b , 1/H/ t ‐Bu) via standard sequences involving intermediate phosphonium salts ( 4a , b and 7a ; TfOH/CH 2 Cl 2 or HBF 4 /chlorobenzene, then PR 2 H, then t ‐BuOK; 10a , b , Ph 3 C + X − , then PR 2 H, then t ‐BuOK). These are converted to the title phosphine oxides (η 5 ‐C 5 R 5 )Re(NO)(PPh 3 )((CH 2 ) n P(O)R' 2 ) ( n /R/R’ = 1a , 0/H/Ph; 1b , 0/H/ t ‐Bu; 8a , 0/Me/Ph; 11a , 1/H/Ph; 11b , 1/H/ t ‐Bu) using PhIO, Me 3 SiOOSiMe 3 or (for 8a ) air. The enantiopure methyl complex ( S )‐ 2 is similarly converted to ( R )‐ 4a , b , ( R )‐ 8a , and ( S )‐ 11a , b . The crystal structures of 1a , ( S )‐ 1a ·(H 2 O) 0.5 , ( S )‐ 8a , and 11a ·CHCl 3 are determined. The basicities of PO‐containing species play key roles in certain enantioselective catalytic reactions. Those of the title and related compounds are computed by DFT ( i.a. 8b > 8a > 1b > 1a > 11b > 11a > t ‐Bu 3 PO > Ph 3 PO). The enhancements in the rhenium‐containing species are rationalized.